Conventionally, in order to reduce noise of an air conditioner during operation, such as blower sound, it has been proposed to include an active noise control mechanism (also called active noise cancellation) for superimposing a canceling sound wave which is in antiphase to noise to cancel the noise.
For example, in Patent Literature 1, a microphone and a flat plate speaker are placed in a predetermined space within an indoor unit of an air conditioner, and the flat plate speaker radiates a sound wave having substantially the same frequency and substantially the same sound pressure level as, and being in substantially antiphase to, noise at a specific frequency detected by the microphone, to thereby perform noise cancellation effectively.
Further, Patent Literature 2 discloses a technology in which operation sound detecting means, reference operation sound setting means, and correction sound emitting means are provided and, when a temporal fluctuation amount of operation sound is larger than a predetermined level, the correction sound emitting means compares the operation sound to a reference operation sound set by the reference operation sound setting means, and radiates a correction sound so that a combined sound obtained when the correction sound is combined with the operation sound becomes the reference operation sound.
Still further, Patent Literature 3 discloses a technology in which a plurality of microphones, controllers, and speakers are provided to constitute independent control systems, respectively, and the microphones and the speakers are disposed at predetermined intervals along an axial direction of a transverse fan.